Highly fuel efficient gas burning water heaters are often provided with powered or forced draft combustion systems. Such combustion systems typically employ a blower for forcing the gas/air combustion mixture to the burner and into the combustion chamber of the heating appliance. High efficiency power assisted water heating appliances are disclosed in U.S. Pat. No. 4,766,883 to Cameron et al and in U.S. Pat. No. 5,085,579 to Moore, Jr. et al, which patents are assigned to the same assignee as that of the present application. The disclosures of U.S. Pat. Nos. 4.766,883 and 5,085,579 are incorporated herein by reference.
Moore 5,085,579 discloses a water heating apparatus in which a combustible gas/air mixture is introduced into a blower which moves the mixture under pressure into a vertically extending tubular burner within a closed combustion chamber contained within a tank containing water. The products of combustion exit the combustion chamber and pass through a helical tube of several turns within the body of water. The heat of combustion is extracted from the products of combustion by conduction through the walls of the combustion chamber and the helical exhaust tube. A high efficiency water heater thereby results.
The heated water from the water heater may alternatively be used to heat the air of a home or building by piping the hot water to a heat exchanger contained within the ducts of the home ventilation or heating system.
Large amounts of energy can be generated by burning a pressurized mixture of fuel and air. Furthermore, as the burner and exhaust tubes are almost entirely surrounded by water to be heated, most of the energy generated by the burner is quickly transferred to the surrounding water. While this configuration results in a highly efficient water heater, it can also create problems which decrease the theoretical efficiency of the system.
Heat flows so rapidly from the burner and exhaust tubing to the surrounding water that some regions within the tank may be heated to a higher temperature than is called for before a thermostat can deactivate the burner. In some cases, the heat may be sufficient to vaporize portions of the water resulting in the generation of steam. Overheating the water is inefficient because extra fuel is consumed to heat the water to an excessive temperature. It can also cause noise as small bubbles of vapor form and collapse. Moreover, when steam is produced, it must be vented to prevent pressure from building up within the tank. This releases energy to the surroundings which could have been used to heat water. The overheating and vaporization of water both prevent a forced draft combustion system from operating at its maximum efficiency.
These problems are aggravated by the fact that the hottest water in a water tank is located near the top of the tank and lower temperature water forms a layer near the tank bottom due to the difference of densities of hot and cold water. This is normally a desirable occurrence for it allows hot water to be drawn from the top of the tank without being mixed with colder water from the supply line.
A high capacity burner of the type contemplated herein can heat cold water rapidly enough to overheat or even vaporize a portion of the water in the tank.
The rate at which hot water is drawn from the tank determines the rate at which cool water in the tank must be heated and, hence the amount of energy that the burner must produce. If the water in the tank is heated too quickly, regions of water in the tank may overheat or vaporize. If the water is heated too slowly, hot water will not be available to replace the water being withdrawn from the tank. Thus, a water heater used to supply large quantities of hot water must heat water more rapidly than a water heater which is required to produce a lesser quantity of hot water. A purchaser of a water heater should, accordingly, select a model adequate to supply maximum normal hot water requirements.
Water heaters are generally available in a limited number of different sizes. This often means obtaining an overcapacity model that heats water at a faster rate than needs require. Use of a water heater which heats water more quickly than necessary to replace hot water being used can contribute to the overheating and vaporization problems mentioned above. It is not practical, however, to stock dozens of different models each suited to a narrow range of supply rates; therefore, the inefficiencies associated with using a system which heats at too high a rate have been impractical to avoid.